In recent years, rechargeable lithium ion batteries have been greatly expected in the industry for use in mobile applications because of its performance providing high capacity and high energy density. These batteries are composed of a positive electrode plate, a negative electrode plate, and a separator (such as an insulating porous polymer film) disposed between the positive electrode plate and the negative electrode plate.
In general, the positive electrode plate and the negative electrode plate are formed by coating and drying on a current collector a paint prepared by mixing an active material (a positive electrode active material or a negative electrode active material), an electrical conduction enhancer, a binder, and a solvent. On this occasion, for the purpose of increasing the discharge capacity of the rechargeable lithium ion battery, these electrode plates (positive electrode plate and negative electrode plate) are generally rolled to increase the active material density (weight of active material contained in electrode plate per unit volume of the plate). As the active material density increases in the electrode plate, the capacity of the rechargeable lithium ion battery increases.
The rechargeable lithium ion battery has a problem of lowering its cycle life due to buckling of the electrode plates that is caused by repeated expansion and shrinking of the active material in the course of charging and discharging. In order to address this problem, there is disclosed an art of incorporating resin particles in the electrode plates (refer to, for example, Japanese Patent Laid-Open Publication No. 2003-168438 and Japanese Patent Laid-Open Publication No. 2001-185152). In the art disclosed in Japanese Patent Laid-Open Publication No. 2003-168438, the resin particles function as a lubricating agent and allow the active material to move so as to absorb the expansion thereof, thereby preventing the buckling of the electrode plate. Further, in the art disclosed in Japanese Patent Laid-Open Publication No. 2001-185152, an elastic structure having a hollow part functions as a cushion material and can prevent the buckling of the electrode.
Furthermore, for the purpose of increasing the rate characteristics of the rechargeable lithium ion battery and preventing the growth of lithium dendrites that cause internal short circuit, there is disclosed an art (refer to, for example, Japanese Patent No. 3581474) of using an active material formed in the shape of a hollow particle and filling therein an electrical conduction enhancer. By filling the electrical conduction enhancer in the active material having such a hollow structure mentioned above, the conductivity can be increased and the rate characteristics can be enhanced. In addition, in the case where the active material contained in the negative electrode plate is in the form of a porous hollow particle made of carbon or a metal oxide, lithium that deposits on charging does not come into direct contact with an electrolyte solution, thereby preventing the dendritic growth of lithium.
In the rechargeable lithium ion batteries, in order to address problems caused by high capacity and high energy density, for example, temperature elevation due to internal short circuit of the battery, various kinds of countermeasures have been undertaken to improve the separator. As a characteristic requested for the separator, there may be mentioned shutdown characteristics and a resistance against short circuit. “Shutdown” means that a porous polymer film (separator) is fused and the pores of the film are closed so as to shutdown current when the battery temperature is elevated by troubles such as over-charging or short circuit inside or outside of the battery. “Short circuit” means that the temperature of the shutdown battery is further elevated and the separator is fully fused, resulting in opening of a hole or the separator is shrunk so that the positive electrode plate comes into direct contact with the negative electrode plate. When such short circuit occurs, a large current is passed across the positive electrode plate and the negative electrode plate, so that the battery is heated.
As describe above, in the rechargeable lithium ion batteries, it is important that both shutdown characteristics and resistance against short circuit be satisfied. As an art that satisfies both of these, there is disclosed an art that provides heat resistance by using a heat resisting nitrogen-containing aromatic polymer and ceramic powder, and prevents thermal runaway by incorporating a thermoplastic resin that melts at 260° C. or lower (refer to, for example, Japanese Patent Laid-Open Publication No. 2000-030686).
Further, for the purpose of attaining high current discharging without lowering capacity, there is disclosed an art of incorporating in a positive electrode hollow particles capable of holding an electrolyte solution, although the art relates to a rechargeable nickel metal hydride battery (refer to, for example, Japanese Patent Laid-Open Publication No. 2002-198043). In this way, by incorporating the hollow particles in the positive electrode, the electrode area can be increased without reducing the volume of the electrolyte solution, thereby attaining a high current discharging without lowering capacity.